Generally a laminated glass consisting of a pair of glass panels and an interlayer film sandwiched between the panels is laudable in safety because even on breakage its fragments do not scatter so that it is in broad use for the glazing of road vehicles such as automobiles and of buildings, among other uses.
Among such interlayer films available for laminated glass use, an interlayer film made of a poly(vinyl butyral) resin plasticized by addition of a plasticizer has excellent adhesion to glass, high tensile strength, and good transparency so that a laminated glass fabricated by using this interlayer film is particularly suited for the glazing of automobiles.
Generally, the sound insulation performance is expressed in terms of frequency-dependent transmission loss, and JIS A 4708 specifies the transmission loss for each sound insulation grade by a given value over the frequency range of 500 Hz and up as indicated by a solid line in FIG. 1. Meanwhile, the sound insulation performance of glass sheet is considerably sacrificed by a coincidence effect in the frequency region around 2000 Hz as shown by a wavy line in FIG. 1.
The valley of the wavy line in FIG. 1 corresponds to a slump in sound insulation performance due to this coincidence effect and indicates that material does not exhibit the required sound insulation performance.
Incidentally, the coincidence effect is a phenomenon such that when a sound wave enters a glass sheet, a transverse wave propagates on the glass surface owing to the rigidity of the glass and inertia, and the resulting resonance between the transverse wave and the incident sound wave causes a transmission of sound.
The conventional laminated glass is satisfactory in the protection against scattering of fragments but, in the aspect of sound insulation performance, is not free from deterioration in this performance due to said coincidence effect in the frequency band around 2000 Hz and, therefore, an improvement in this aspect has been awaited.
Meanwhile, from the equivalent loudness curve, the human ear is known to have a remarkably high sensitivity over the range of 1000 to 6000 Hz as compared with the other frequency range and this indicates that it is of great importance to a satisfactory sound insulation to get rid of the above slump in sound insulation performance due to the coincidence effect.
In order that the sound insulation performance of a laminated glass may be improved, it is necessary to attenuate the above coincidence effect to prevent said slump in the minimal part of transmission loss which arises from the coincidence effect (hereinafter the transmission loss in this minimal part will be referred to as TL value; FIG. 1).
As a means for preventing the slump in TL value, there has been proposed many measures, such as increasing the mass of laminated glass, a multi-layer glass construction, division of the glass area, and improving the glass panel-supporting structure, among others. However, these means are not only more or less unsatisfactory in effect but not available at commercially acceptable cost.
The requirements about sound insulation performance are getting more and more rigorous lately and taking architectural glazing materials as an example, the glass is required to have high sound insulation around room temperature. Thus, the required good sound insulation performance is such that the temperature corresponding to the highest sound insulation performance as determined by plotting transmission loss (TL value) against temperature (Maximum sound insulation temperature=TLmax temperature) is in the neighborhood of room temperature and that the maximum value of sound insulation performance (maximum sound insulation value=TLmax value) itself is large. The same is true of road vehicles. Thus, including the wind-cutting sound at high-speed driving, vibrations from the engine assembly, etc., the number of sources of noise calling for a high level of sound insulation has been on the increase.
Furthermore, laminated glasses in these applications are actually exposed to a marked fluctuation of ambient temperature from a low temperature region to a high temperature region so that a satisfactory sound insulation performance is required not only in the neighborhood of room temperature but also over a broad temperature range.
However, the conventional laminated glass fabricated by using an interlayer film made of plasticized poly (vinyl butyral)resin has the disadvantage that its maximum sound insulation temperature is higher than room temperature and its sound insulation performance in the neighborhood of room temperature is poor.
Moreover, if an attempt is made to insure a satisfactory sound insulation performance, the interlayer film has to be soft enough so that when assembled with glass panels to fabricate a laminated glass, such troubles as panel shear and foaming tend to take place.
As the prior art interlayer film designed to improve the sound insulation performance of a laminated glass, Japanese Kokai Publication Hei-02-229742 discloses an polymer layer with glass transition temperature of not higher than 15° C., for example an interlayer comprising a laminate consisting of a vinyl chloride-ethylene-glycidyl methacrylate copolymer film and a plasticized poly(vinyl acetal) film.
This interlayer, however, is not only incapable of showing a sound insulation performance over Ts-35 on the sound insulation grade according to JIS A 4706 but also limited in the temperature range in which sound insulation performance is exhibited, failing to show a satisfactory sound insulation-performance over a broad temperature range.
There has also been proposed an interlayer film for a laminated glass which comprises a poly (vinyl acetal) resin with an acetalization degree of 60 to 85 mol % and an acetyl group content of 8 to 30 mol %, the combined total of said acetalization degree and acetyl group content being not less than 75 mol %, and a plasticizer which, in the presence of the resin, shows a cloud point of not higher than 50° C. This interlayer film has certainly been improved in the aspects of sound insulation performance and temperature dependence of the performance but because the film is soft, it has the drawback that, when assembled with glass panels to fabricate a laminated glass, such troubles as panel shear and foaming tend to occur.
Japanese Kokai Publication Sho-51-106190 proposes a composition having a damping function over a broad temperature range as fabricated by laying up two or more kinds of resins varying in glass transition temperature. It is stated that this composition has an improved damping function over a broad temperature range. However, it is not obvious from the description whether this composition ever has properties required of laminated glass such as the sound insulation properties and transparency, and, moreover, this composition does not satisfy the requirements necessary for a safety glass, namely high impact energy absorbency and prevention of fragment scattering in the event of glass breakage.
Japanese Kokai Publication Hei-04-254444 proposes an interlayer fabricated by laminating a film consisting of a poly(vinyl acetal) containing acetal groups of 6 to 10 carbon atoms and a plasticizer with a film consisting of a poly(vinyl acetal) containing acetal groups of 1 to 4 carbon atoms and a plasticizer. This interlayer has a definitely improved sound insulation performance with little temperature-dependent variation but these improvements are still insufficient.
Thus, none of the above prior art interlayer films have satisfactory physical properties or are fully qualified to provide for laminated glass products showing a fully satisfactory sound insulation performance over a broad temperature range.